1. Field of the Invention
The present invention relates to a high-frequency signal transmission line, and more particularly to a thin flexible high-frequency signal transmission line.
2. Description of the Related Art
In a high-frequency signal transmission line of a tri-plate stripline structure wherein a signal line is sandwiched between an upper and a lower grounding conductor, one way of decreasing the resistance to high-frequency waves is to increase the width of the signal line. Specifically, by increasing the width of the signal line, the surface area of the signal line is increased, and accordingly, the areas of the grounding conductors opposed to the signal line are increased. Therefore, the resistance of the signal line to high-frequency waves can be reduced.
However, the increase in the areas of the grounding conductors opposed to the signal line caused by the increase in width of the signal line leads to an increase in capacitances among the signal line and the grounding conductors. Also, the increase in width of the signal line leads to a decrease in inductance of the signal line. Still in this state, in order to achieve predetermined characteristic impedance (for example, 50Ω) of the high-frequency signal transmission line, the distances among the signal line and the grounding conductors are increased so that the capacitances can be reduced. However, the increase in distances among the signal line and the grounding conductors leads to an increase in thickness of the high-frequency signal transmission line, which is an obstacle to achievement of a flexible high-frequency signal transmission line.
In order to avoid this problem, it is possible that the signal line is arranged so as not to be opposed to a grounding conductor. This arrangement is described referring to the drawings. FIG. 16A is a top view of a high-frequency signal transmission line 500, viewed from a lamination direction, and FIG. 16A shows that a signal line 502 is not covered by a grounding conductor 504. FIG. 16B is a cross-sectional view of the high-frequency signal transmission line 500.
The high-frequency signal transmission line 500 includes, as shown in FIGS. 16A and 16B, a signal line 502 and grounding conductors 504 and 506. The signal line 502 is a linear conductor. The grounding conductor 506 is located at a lower level than the signal line 502 with respect to the lamination direction and is opposed to the signal line 502 via a dielectric layer. The grounding conductor 504 is located at an upper level than the signal line 502 with respect to the lamination direction and has an opening. The signal line 502 is located within the opening when viewed from the top in the lamination direction.
In the high-frequency signal transmission line 500 shown in FIGS. 16A and 16B, the signal line 502 and the grounding conductor 504 are not overlaid with each other when viewed from the lamination direction. Therefore, the capacitance generated between the signal line 502 and the grounding conductor 504 in the high-frequency signal transmission line 500 is smaller than that generated between a signal line and a grounding conductor in a high-frequency signal transmission line wherein the signal line and the grounding conductor are opposed to each other. In the high-frequency signal transmission line 500, therefore, it is possible to shorten the distance between the signal line 502 and the grounding conductor 504. Consequently, the thickness of the high-frequency signal transmission line 500 can be decreased, and the high-frequency signal transmission line 500 can be made flexible so as to be usable while bent.
However, the high-frequency signal transmission line 500 has a problem in that unnecessary radiation from the signal line 502 occurs. The signal line 502 is not covered by the grounding conductor 504. For this reason, the electromagnetic field generated by a current flow in the signal line 502 is radiated outward from the high-frequency signal transmission line 500 through the opening, and unnecessary radiation occurs. Moreover, the signal current partly leaks out as unnecessary radiation, which causes another problem of an increase in insertion loss of the signal current in the high-frequency signal transmission line 500.
As a high-frequency signal transmission line suggested to solve the problems above, for example, a flexible board disclosed by Japanese Patent Laid-Open Publication No. 2007-123740 is known. FIG. 17 is a top view of the flexible board 600 described in Japanese Patent Laid-Open Publication No. 2007-123740, viewed from a lamination direction.
The flexible board 600 includes a signal line 602 and a grounding layer 604. The signal line 602 is a linear conductor. The grounding layer 604 is located at an upper level than the signal line 602 with respect to the lamination direction via a dielectric layer. Although not shown in FIG. 17, another grounding layer is located at a lower level than the signal line 602 with respect to the lamination direction. In the flexible board 600, a plurality of openings 606 are provided in the grounding layer 604. The openings 606 are rectangular and are aligned over the signal line 602 in the extending direction of the signal line 602. Therefore, as viewed from the top of the lamination direction, the signal line 602 is partly overlaid with the grounding layer 604. Thus, since the signal line 602 is partly covered by the grounding layer 604, the flexible board 600 reduces unnecessary radiation from the signal line 602.
With respect to the flexible board 600, however, there is still a problem in that it is difficult to suppress unnecessary radiation while maintaining flexibility and realizing predetermined characteristic impedance of the whole signal line 602. This problem is described in more detail below.
The signal line 602 has portions covered by the grounding layer 604 (covered by bridges 607) and portions exposed by the openings 606 that are arranged alternately in the extending direction thereof. In order to reduce unnecessary radiation from the signal line 602, it is necessary that the openings 606 have small dimensions X1 and Y, wherein X1 is a dimension in the extending direction of the signal line 602, and Y is a dimension in a direction perpendicular to the extending direction of the signal line 602. However, if the dimensions X1 and Y of the openings 606 are set smaller, these exposed portions of the signal line 602 will exhibit too small impedance, whereby the characteristic impedance of the whole signal line 602 will be small. On the other hand, if the dimension X1 is set larger to make the dimension X2 of the bridges 607 smaller, the characteristic impedance will be greater, but unnecessary radiation through the openings 606 will be increased. Even if only the dimension Y of the openings 606 is set larger, the characteristic impedance will be too small because the other grounding conductor is provided at a lower level than the signal line 602 in the lamination direction. Therefore, in order to avoid this, the thickness of the flexible board will need to be increased. Thus, it is necessary to design the dimensions X1, X2 and Y simultaneously so as to achieve predetermined characteristic impedance and to suppress unnecessary radiation.
From an industrial viewpoint, it is a preferable method to determine the dimension Y including an allowance to eliminate the possibility that the openings 606 may be displace from the signal line 602 due to lamination errors (for example, Y=the width of the signal line+200 μm) before determining the dimensions X1 and X2. However, actually, the dimension X2 depends on the limitation of industrial thin line processing technology and is determined to be, for example, 200 μm. Then, the minimum value for the dimension X1, which determines the characteristic impedance, is automatically determined. The minimum value for the dimension X1 corresponds to the wavelength of a signal with the highest frequency the signal line is transmittable, and accordingly, when the minimum value for the dimension X1 is large, it means that the signal line has a poor transmission characteristic. Instead, when the dimension X2 is determined before the dimensions X1 and Y, the openings 606 will be designed such that the dimension X1 will be greater than X2 and such that the dimension Y will be, for example, about 1 mm. In this case, the length of the diagonal line of each of the openings 606 determines the frequency characteristic of unnecessary radiation and the frequency characteristic of transmission loss of the signal line.
The width X2 of the bridges 607 of the grounding layer 604 is very small and for example, 100 μm, and the length Y (corresponding to the width of the openings 606) of the bridges is about 1 mm as mentioned above. In this case, inductance occurs at the bridges. By arranging the signal line 602 and the grounding layer 604 as close as possible to each other, the high-frequency signal transmission line can be made as flexible as possible. By narrowing the bridges, for example, by reducing the width X2 of the bridges 607 from 100 μm to 50 μm and further to 30 μm, the lamination of the signal line 602 and the grounding conductor 604 can be made thinner. However, the reduction in width X2 of the bridges 607 also makes the inductance occurring at the bridges 607 greater, thereby causing fluctuation of the grounding potential of the grounding conductor 604. That is, great inductance unnecessarily occurs between the signal line 602 and the bridges X2, which degrades the grounding effect of the grounding conductor 604. Thereby, great unnecessary radiation may occur, and the grounding current concentrates on the bridges 607, which may cause a great loss. In sum, the trouble is caused by two problems. One is as follows: since the bridges 607 are thin and narrow electrodes, a high-frequency current is generated in the center portion of each of the bridges 607 by electromagnetic coupling between each of the bridges 607 and the signal line 602, and due to the high-frequency current, inductance unnecessarily occurs. The other is as follows: the unnecessary inductance is magnetically coupled with inductance that occurs due to a current flow in the signal line 602, which further causes mutual inductance, thereby making the unnecessary inductance at the bridges 607 greater.